package/*: test for BR2_PACKAGE_LIBGUDEV for dependency

Make the case of adding libgudev as a dependency more consistent with all
other dependency packages. The <pkg>_DEPENDENCIES variable sets the build
order which is conceptually independent of the underlying reason for that
dependency.

Also, remove the comments in hidapi.mk and udisks.mk. These comments explain
why BR2_PACKAGE_LIBGUDEV is selected, so the comments in corresponding
Config.in files are enough.

Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>